1. Field of the Invention
This invention relates to the field of liquefied natural gas (LNG) gas conditioning processes, and in particular to the recovery of liquefied petroleum gas (LPG) containing propane and heavier components or natural gas liquids (NGL) containing ethane and heavier components (C2+) from LNG.
2. Description of the Related Art
Natural gas is often produced at remote locations that are far from pipelines. An alternative to transporting natural gas through a pipeline is to liquefy the natural gas and transport it in special LNG tankers. Natural gas may be liquefied by compressing it or by cooling it. An LNG handling and storage terminal is necessary to receive the compressed or cooled liquefied natural gas and revaporize it for use. The re-vaporized natural gas may then be used as a gaseous fuel.
A typical LNG handling, storage and revaporization facility, such as the one shown in FIG. 1, may include an incoming stream of LNG 10, a ship vapor return blower 12, LNG storage and send out pumps 14, a boil off gas compression and condensation unit 16, LNG booster pumps 18, LNG vaporizers 20, and an outgoing stream to a natural gas pipeline 22.
Natural gas in general, and LNG in particular, is usually comprised mostly of methane (C1). Natural gas may also, however, contain lesser amounts of heavier hydrocarbons such as ethane (C2), propane (C3), butanes (C4) and the like, which are collectively known as propane plus, or C2+.
Natural gas shipped over a pipeline, for example, may need to conform to a particular specification for heating value. Since various hydrocarbons have various heating values, it is often necessary to separate some or all of the heavier hydrocarbons from the methane in the LNG so that the gaseous fuel resulting from vaporizing the LNG has the right heating value. Furthermore, heavier hydrocarbons have a higher value as liquid products (for use as petrochemical feed stocks, for example) than as fuel, and it is thus often desirable to separate the heavier hydrocarbons from the methane.
A heating value specified by a pipeline may change over time. Some of the customers of the pipeline may be satisfied with lean natural gas, while others may be, willing to pay for higher heating values. A natural gas recovery system in which all incoming LNG passes through a single point of entry, or even a plurality of symmetrical points of entry, may be unable to blend heating values to suit various pipeline specifications.
Fractionation units, such as distillation or de-methanation units, may use heat exchangers to recover some of the heat left in the product stream and use it to heat the incoming feed streams. In some cases there is insufficient heat in the product for a particular hydrocarbon to be effectively separated. In some cases there is, a need to boost the heat of an incoming stream to more effectively separate a particular hydrocarbon. In some cases a middle feed, for example, receives adequate heat from the product stream while a bottom feed, for example, is too cool, and requires some further energy input to effectively separate some particular hydrocarbon.